1. Field of the Invention
The invention relates in general to a method and apparatus for fluidizing particulate material during delivery to and filling cavities in a uniform and consistent manner during successive filling operations. The invention relates in particular to powder delivery systems for delivering particulate material and filling closed cavities, such as die cavities of die-casting machines, and for filling open cavities, such as containers on food packaging production lines.
2. Description of the Related Art
Powder delivery systems are used for delivering particulate material and filling die cavities of, e.g., die-casting machines prior to powder compaction in processes for fabricating consolidated part for automotive, aerospace, micro-electronics, pharmaceuticals, vitamins, etc. Powder typically is gravity-fed from a main hopper to the die cavity by transfer through a flexible tube to a feedshoe or fillshoe, which deposits powder into the die cavity. The process of depositing powder in the die cavity is called xe2x80x9cdie filling.xe2x80x9d
The process of powder delivery and filling by gravity is the most common way of delivering powder and filling a die cavity. The feedshoe is pushed forward between the surface of the die cavity and a top punch, and positioned on top of the die cavity. Depending on powder flow characteristics, cavity shape and size, type of die-casting machine, and filling method, the feedshoe is typically mechanically vibrated while on top of the die cavity to loosen the powder, break any clumps and ensure that the die cavity is full before the feedshoe is retracted.
There are several problems associated with prior art processes for powder delivery and filling of a die cavity.
One such problem is variation in filling conditions during successive filling operations, i.e., from part to part. Variations in filling conditions from part to part result in variations in weight from part to part, and non-uniform fill of the die cavities. A non-uniform fill results in variations in density between the front and back of the part and in distortion of the part during sintering. Typically, part specifications include limits on acceptable variations in part weight and on acceptable variations in density within the part. Parts that do not meet the specifications are rejected.
The variations in filling conditions from part to part are due, in part, to variations in head pressure, clumping, surge, and dead zones of material within the feedshoe. The head pressure is due to the powder in the feedshoe, the flexible tube and the main hopper. As a result of the powder delivery system design, wherein the flexible tube is typically connected to the backside of the feedshoe, the powder in the back of the die cavity is subjected to a higher head pressure than the front of the die cavity.
During operation, the feedshoe is pushed forward and centered above the die cavity and then retracted before compacting. The movement of the feedshoe across the die cavity results in the back of the die cavity being subjected to the higher head pressure for a longer period of time than the front of the die cavity. These effects combine to produce variations in part density between the back and the front of the part, which result in distortion during sintering.
Clumping and surge of the powder within the feedshoe and flexible tube contributes to non-uniform filling of die cavities. Mechanical shaking of the feedshoe above the die cavity can break clumping in the powder and improve fill uniformity but is not consistent during successive filling operations.
A further problem results from fine powders/particulate materials, which do not have good flow characteristics, thus posing a serious problem for the die filling operation. Lubricants are added to reduce interparticle friction and improve flowability, thereby requiring an energy intensive delubing cycle after compacting to remove all added lubricants.
A further problem is that mechanical shaking of the feedshoe causes segregation of fine powders/particulate materials from coarse powders/particulate materials resulting in a loss of uniformity in particle size distribution and chemical composition. This powder segregation results in powders with different apparent densities and chemical composition being filled in the die cavity during successive powder filling operations.
A solution to these and other problems is needed. Such solution is provided by the novel invention recited herein.
The invention contemplates supplying a technique and apparatus for delivering particulate material and filling cavities in a uniform and consistent manner during successive filling operations. In preferred embodiments, the invention provides a powder delivery system for delivering particulate material and filling closed cavities, such as the closed die cavities of die-casting machines; or for filling open cavities, such as open containers on food packaging production lines.
Die Casting and Sintering
The three basic steps for producing parts by the press and sinter process are mixing, compacting and sintering. In step one, mixing, elemental or prealloyed powders are mixed with lubricants or other alloy additions to produce a homogeneous mixture of ingredients. The lubricant reduces interparticle friction and improves the flow characteristics of the powder mixture. In step two, compacting, mixed powder is fed into a precision die on a die-casting machine and is compacted. Compacting the loose powder produces a xe2x80x9cgreen compactxe2x80x9d which has the size and shape of the finished part when ejected from the die. In step three, sintering, the green compacts are placed on a wide-mesh belt and slowly moved through a controlled atmosphere furnace. The parts are heated to below the melting point of the base material, held at the sintering temperature, and then cooled.
Tabletting and Dry Compaction
The production of pharmaceutical preparations, e.g., vitamins, or tablets containing an active medicament in a carrier or other suitable excipient, requires precise and homogeneous mixation techniques. Similarly, candies usually must have an acceptable hardness, mouthfeel, and duration within the mouth. These characteristics can depend in part upon the homogeneity of the composition in the tablet form.
Dry powder, or a semidry paste, is placed within a tablet mold and subjected to pressure. The amount of pressure usually determines the hardness of the tablet, and consequently its lifetime within the mouth (subject, of course, to chewing). It is important for dosage amount and appearance that the powder feed correctly into the tabletting machine.
The Closed Cavity Device
In one embodiment, the invention provides a method and apparatus for powder delivery and filling of a closed cavity, such as a die cavity of a die-casting machine. The apparatus includes a mini-hopper, a transport device, a delivery chute and a gas control unit.
The mini-hopper has a porous distributor plate for partitioning the mini-hopper into a first partition in which the bed of particulate material is stored and a second partition separate from the first partition and in communication with the first partition via the porous distributor plate. An inlet port is provided for receiving a compressed gas in the second partition at a low pressure, whereby only the bottom surface of the bed of particulate material becomes fluidized by migration of the compressed gas through the porous distributor plate and into the first partition.
The transport is connected to the side of the mini-hopper and delivers powder/particulate material from the mini-hopper to the delivery chute. The transport has a porous distributor plate for partitioning the transport into a first partition in which the particulate material flows and a second partition separate from the first partition and in communication with the first partition via the porous distributor plate.
The delivery chute can function as the powder discharge unit directly above the die cavity. Fluidizers are embedded in the delivery chute to ensure that powder is fluidized before filling the die cavity. The delivery chute is customized to part shape to optimize fill performance for individual parts or a family of parts, depending on part size and shape complexity.
An in-line dryer may be provided to remove moisture from the gas supply, while an in-line filter may be used to remove solid impurities in the gas supply. The gas control unit can include three independent pressure regulators, located in a separate housing, and three pneumatic solenoids, which are used to regulate the flow of gas to each segment of the fillshoe independently. The solenoids are preferably timed to control fluidization of the powder over the die cavity.
The apparatus in one embodiment provides for venting of gas in each segment of the fillshoe to prevent build up of pressure within the system and in the die cavity, which will prevent uniform, complete and consistent filling of die cavities. The apparatus also preferably provides for collection of any fine powder particles that may escape through the venting screens.
The apparatus in a particularly preferred embodiment has a low profile, fits on a compacting press, and can be pushed into position between the die surface and top punch of a die-casting machine during die filling operation.
The apparatus for filling closed cavities is preferably easily movable. The apparatus is pushed forward into position above a die cavity for filling and then retracted after filling and before compacting. The low profile apparatus for filling closed cavities can be positioned between the die surface and top punch of a die-casting machine.
The Open Cavity Filling Device
The invention, in another preferred embodiment, provides a method and apparatus for powder delivery and filling of an open cavity, such as a container on food packaging production lines. The apparatus for filling open cavities is preferably stationary, although it may be moved for adjustment purposes or to accommodate packagings of various sizes on a continuous production line.
The apparatus in one embodiment includes a mini-hopper, a transport, a delivery chute and a gas control unit.
The mini-hopper can function as an intermediate storage unit for powder and is configured to receive powder from the main hopper. The mini-hopper has specially designed porous distributor plates for partitioning the mini-hopper into a first partition in which the bed of particulate material is stored and a second partition separate from the first partition and in communication with the first partition via the porous distributor plate.
An inlet port is provided for receiving a compressed gas in the second partition at a low pressure, whereby only a layer of particulate material next to the porous distributor plates becomes fluidized by migration of the compressed gas through the porous distributor plates and into the first partition.
The transport delivers powder/particulate material from the mini-hopper to the delivery chute. The transport has a porous distributor plate for partitioning the transport into a first partition in which the particulate material flows and a second partition separate from the first partition and in communication with the first partition via the porous distributor plate. An inlet port is provided for receiving a compressed gas in the second partition at a low pressure, whereby only the bottom layer of the particulate material becomes fluidized by migration of the compressed gas through the porous distributor plate and into the first partition.
The delivery chute preferably has a center fluidizer which is used to regulate powder flow and meter the amount of powder. This can be accomplished, e.g., by the use of a timer and turning the gas flow on and off. The delivery chute can act as a fluidized powder valve by preventing powder flow once the gas is turned off.
The gas control unit is used to control the gas moisture content and regulate powder fluidization and powder flow in relation to the movement of the production/filling line.
In a preferred embodiment, an in-line dryer removes moisture from the gas supply. An in-line filter is also preferably used to remove solid impurities in the gas supply. The gas control unit is used to control the gas moisture content and regulate powder fluidization in relation to the movement of the production line.
The gas control unit in one embodiment includes three independent pressure regulators, located in a separate housing, and three pneumatic solenoids, which are used to regulate the flow of gas to each segment of the fillshoe independently. The solenoids are timed to control fluidization of the powder over the open container on a production line. When the gas to the fluidized powder valve is turned on, particulate material flows into the open container. When the gas to the fluidized powder valve is turned off, the flow of particulate material is cut-off immediately.
A timer is preferably used to regulate the time at which the gas to the fluidized powder valve is turned on and off, providing an accurate way of metering particulate material for filling a container or an open cavity.
It is therefore an object of this invention to provide a new method and apparatus for powder filling of a die cavity.
It is a further object of this invention to provide an apparatus with a low profile that can be fitted on a die-casting machine between the die surface and the top punch of the die-casting machine.
It is a further object of the invention to provide a method and apparatus for powder delivery and consistent filling of a die cavity from the feedshoe during successive filling operations.
It is a further object of the invention to provide a method and apparatus for powder delivery and consistent filling of a die cavity which improves uniformity of fill of the die cavity from the feedshoe.
It is a further object of the invention to provide a method and apparatus for powder delivery to a die cavity which reduces or eliminates segregation of fine particles from coarse particles and which maintains uniformity in chemical composition.
It is a further object of the invention to provide a method and apparatus for powder delivery and filling of a die cavity which enhances the flowability of fine powders/particulate materials.
It is a further object of the invention to provide a method and apparatus for powder delivery and filling of a cavity for forming tablets, which apparatus enhances the flowability of fine powders/particulate materials.
It is a further object of the invention to provide a method and apparatus for powder delivery and filling of a die cavity which reduces or eliminates the need for using organic binders to adhere the fine particles with the coarse particles to prevent particle segregation.
It is a further object of the invention to provide a method and apparatus for powder delivery to a die cavity, which minimizes or eliminates the need to use a lubricant.
The foregoing and other objects, features, and advantages of the invention will be apparent to the skilled artisan having regard for this disclosure. The examples which follow are intended to serve as illustrative and not by way of limitation.